It is a fact that surround and multi-channel sound tracks are gradually replacing stereo as the preferred standard of sound recording. Today, many new audio devices are equipped with surround capabilities. Most new sound systems sold today are multi-channel systems equipped with multiple speakers and surround sound decoders. In fact, many companies have devised algorithms that modify old stereo recordings so that they will sound as if they were recorded in surround. Other companies have developed algorithms that upgrade older stereo systems so that they will produce surround-like sound using only two speakers. Stereo-expansion algorithms, such as those from SRS Labs and Spatializer Audio Laboratories, enlarge perceived ambiance; many sound boards and speaker systems contain the circuitry necessary to deliver expanded stereo sound.
Three-dimensional positioning algorithms take matters a step further seeking to place sounds in particular locations around the listener, i.e., to his left or right, above or below, all with respect to the image displayed. These algorithms are based upon simulating psycho-acoustic cues replicating the way sounds are actually heard in a 360° space, and often use a Head-Related Transfer Function (HRTF) to calculate sound heard at the listener's ears relative to the spatial coordinates of the sound's origin. For example, a sound emitted by a source located to one's left side is first received by the left ear and only a split second later by the right ear. The relative amplitude of different frequencies also varies, due to directionality and the obstruction of the listener's own head. The simulation is generally good if the listener is seated in the “sweet spot” between the speakers.
In the consumer audio market, stereo systems are being replaced by home theatre systems, in which six speakers are usually used. Inspired by commercial movie theatres, home theatres employ 5.1 playback channels comprising five main speakers and a sub-woofer. Two competing technologies, Dolby Digital and DTS, employ 5.1 channel processing. Both technologies are improvements of older surround standards, such as Dolby Pro Logic, in which channel separation was limited and the rear channels were monaural.
Although 5.1 playback channels improve realism, placing six speakers in an ordinary living room might be problematic. Thus, a number of surround synthesis companies have developed algorithms specifically to replay multi-channel formats such as Dolby Digital over two speakers, creating virtual speakers that convey the correct spatial sense. This multi-channel virtualization processing is similar to that developed for surround synthesis. Although two-speaker surround systems have yet to match the performance of five-speaker systems, virtual speakers can provide good sound localization around the listener.
All of the above-described virtual surround technologies provide a surround simulation only within a designated area within a room, referred to as a “sweet spot.” The sweet spot is an area located within the listening environment, the size and location of which depends on the position and direction of the speakers. Audio equipment manufacturers provide specific installation instructions for speakers. Unless all of these instructions are fully complied with, the surround simulation will fail to be accurate. The size of the sweet spot in two-speaker surround systems is significantly smaller than that of multi-channel systems. As a matter of fact, in most cases, it is not suitable for more than one listener.
Another common problem, with both multi-channel and two-speaker sound systems, is that physical limitations such as room layout, furniture, etc., prevent the listener from following placement instructions accurately.
In addition, the position and shape of the sweet spot are influenced by the acoustic characteristics of the listening environment. Most users have neither the mean nor the knowledge to identify and solve acoustic problems.
Another common problem associated with audio reproduction is the fact that objects and surfaces in the room might resonate at certain frequencies. The resonating objects create a disturbing hum or buzz.
Thus, it is desirable to provide a system and method that will provide the best sound simulation while disregarding the listener's location within the sound environment and the acoustic characteristics of the room. Such a system should provide optimal performance automatically, without requiring alteration of the listening environment.